1. Field of the Invention
This invention relates to a focus detecting device for use in a single-lens reflex camera or the like.
2. Related Background Art
FIG. 13 of the accompanying drawings shows an example of a conventional focus detecting device of a camera which detects the deviation between two images formed by dividing the pupil of an objective lens and discriminates the focus-adjusted state of the objective lens.
A light flux which has entered through the area 101 of an objective lens 100 passes through a field mask 200, a field lens 300, a stop opening portion 401 and a re-imaging lens 501 and is imaged on an image sensor array P1. Likewise, a light flux which has entered through the area 102 of the objective lens 100 passes through the field mask 200, the field lens 300, an opening portion 402 and a re-imaging lens 502 and is imaged on an image sensor array P2. The pair of optical images formed on the image sensor arrays P1 and P2 are positioned away from each other in the so-called front focus state in which the focus-adjusted state of the objective lens 100 is such that a sharp image of an object is formed forwardly of the predetermined focal plane thereof, and conversely approach each other in the rear focus state in which a sharp image of the object is formed rearwardly of the predetermined focal plane, and the optical images fall in line with each other at a predetermined interval intermediate said two positions when a sharp image of the object is formed on the predetermined focal plane. Accordingly, by photoelectrically converting the respective optical images by the image sensor arrays P1 and P2, and operation-processing these signals to find the amount of deviation between the pair of optical images, the focus-adjusted state of the objective lens 100 can be known.
In such a focus detecting system, reliable focus detection cannot be accomplished unless the optical images formed on the image sensor arrays have a contrast greater than a certain degree. Noting that an object to be photographed has a higher contrast in the horizontal direction than in the vertical direction, it is the usual practice to dispose a pair of image sensors P1 and P2 in the horizontal direction X as shown in FIG. 14A of the accompanying drawings, and effect focus detection by the contrast in the horizontal direction.
There is also known a system in which, in a case where the contrast in the horizontal direction is low and the contrast in the vertical direction is high or a case where a camera is used in its vertical position, a pair of image sensor arrays P1 and P2 and a pair of image sensor arrays P3 and P4 are disposed in the horizontal direction X and the vertical direction Y, respectively, as shown in FIG. 14B of the accompanying drawings, whereby focus detection is made possible for the contrast in either direction.
In such a focus detecting device, where the photo-taking lens has infrared aberration, different focus detection, is effected for light of different wavelengths. More particularly, many photoelectric converting portions of image sensor arrays, for example, are P-N junction type photodiodes, and the sensitivity thereof extends from the visible range to the near-infrared range. Therefore, comparing a case where an object is illuminated by a light source of low color temperature such as a tungsten lamp and a case where the object is illuminated by a light source of high color temperature such as daylight or a fluorescent lamp, different focus detection is effected depending on the infrared aberration of the photo-taking lens. Particularly, where the photo-taking lens is a long-focus lens, the infrared aberration is great, and when focus detection is effected with a long-focus lens mounted on a single-lens reflex camera, the occurrence of focus detection error due to the infrared aberration is substantial, and depending on the light source, there arises a difference as great as several hundreds of microns on the film surface and focus detection cannot be accomplished accurately.
So, in the focus detecting device disclosed, for example, in Japanese Laid-Open patent application No. 62-174710, as shown in FIG. 14A of the accompanying drawings, photosensors M1 and M2 are disposed near the image sensor arrays P1 and P2, respectively, and the proportion of the infrared light in the incident light flux is found on the basis of signals from the photosensors M1 and M2, whereby the result of focus detection found from the outputs of the image sensor arrays P1 and P2 is corrected to thereby suppress the influence of infrared aberration.
Also, in the above-described focus detecting device wherein pairs of image sensor arrays P1, P2 and P3, P4 are disposed in the horizontal and vertical directions, respectively, photosensors M1, M2, M3 and M4 may be disposed for the image sensor arrays P1, P2, P3 and P4, as shown in FIG. 14B. That is, for the horizontal direction, the photosensors M1 and M2 are disposed near the image sensor arrays P1 and P2, respectively, and the proportion of infrared light included in the incident light is detected by the signals from the pair of photosensors M1 and M2, and for the vertical direction, the photosensors M3 and M4 are disposed near the image sensor arrays P3 and P4, respectively, and the proportion of infrared light included in the incident light is detected by the signals from the photosensors M3 and M4.
However, if the focus detecting optical system of FIG. 13 is used and the photosensors are disposed as shown in FIGS. 14A and 14B, the focus detecting area in which the image sensor arrays are disposed and the infrared light detecting area in which the photosensors are disposed do not coincide with each other. That is, in the arrangement of the image sensor arrays P1 and P2 and the photosensors M1 and M2 shown in FIG. 14A, the image IP of the image sensor arrays P1 and P2 formed on the predetermined focal plane of the phototaking lens 101 (objective lens) through the re-imaging lenses 501 and 502 and the field lens 300 and the image IM of the photosensors M1 and M2 vertically deviate from each other as shown in FIG. 15A of the accompanying drawings. This means that the focus detecting area IP and the infrared light detecting area IM do not coincide with each other, and where the object is irradiated with various light sources and the distribution of the infrared light of the light flux entering through the photo-taking lens is not uniform, there is the problem that accurate detection of the infrared light is not accomplished and wrong correction of infrared aberration is effected
Likewise in the case of FIG. 14B, the focus detecting area and the infrared light detecting area deviate from each other as shown in FIG. 15B of the accompanying drawings and the same problem arises. In FIG. 15B, IP1 designates the image of the image sensor arrays P1 and P2 as they overlap each other, IP2 denotes the image of the image sensor arrays P3 and P4 as they overlap each other, IM1 designates the image of the photosensors M1 and M2 as they overlap each other, and IM2 denotes the image of the photosensors M3 and M4 as they overlap each other, and with respect to the horizontal direction, the focus detecting area IP1 and the infrared light detecting area IM1 vertically deviate from each other, and with respect to the vertical direction, the focus detecting area IP2 and the infrared light detecting area IM2 horizontally deviate from each other.